Anesth Analg 2005;101:1844-1846
© 2005 International Anesthesia Research Society
doi: 10.1213/01.ANE.0000184255.43746.89
REGIONAL ANESTHESIA
Signs of Inflammation After Sciatic Nerve Block in Pigs
Wolfgang G. Voelckel, MD*,
Günther Klima, MD
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Anette C. Krismer, MD*,
Christopher Haslinger, BS*,
Karl H. Stadlbauer, MD*,
Volker Wenzel, MD*, and
Achim von Goedecke, MD*
Departments of *Anesthesiology and Critical Care Medicine and Histology, Innsbruck Medical University, Austria
Address correspondence and reprint requests to Wolfgang Voelckel, MD, MSc, DEAA, Department of Anesthesiology and Critical Care Medicine, Innsbruck Medical University, Anichstrasse 35, 6020 Innsbruck, Austria. Address e-mail to wolfgang.voelckel{at}uibk.ac.at.
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Abstract
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Nerve stimulators are widely used to assist with peripheral nerve blocks but do not eliminate the risk of nerve injury. We evaluated the histologic findings 6 h after sciatic nerve block with bupivacaine in pigs. When a motor response was still obtained with a current <0.2 mA (n = 10), the postmortem microscopic evaluation revealed lymphocytes and granulocytes sub-, peri-, and intraneurally in 5 (50%) of 10 pigs. No signs of inflammation were observed when the muscle contraction was achieved with a current between 0.3 and 0.5 mA (P = 0.03). In conclusion, the current required to elicit a motor response, the position of the needle tip, and the subsequent likelihood of nerve damage merit further evaluation.
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Introduction
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Introduction of nerve stimulators for peripheral nerve blocks did not eliminate the risk of persisting neurologic sequelae (1–4). This is not surprising because every puncture of the neurovascular sheath carries the risk of direct nerve damage with the needle’s tip. Although it is possible to cause paresthesia without the ability to produce a motor response (5,6), the rationale of peripheral nerve stimulation is based on the hypothesis that the distance between the needle’s tip and the nerve is reflected by the actual current. Thus, motor responses elicited with a very low current may suggest a direct needle tip contact, and a forceful injection of drugs in this setting may risk nerve damage. There are no studies demonstrating the potential relationship between the electric current required for the desired motor response before injection of the local anesthetic and subsequent alternations of the nerve. Accordingly, we assessed the integrity of the sciatic nerve histologically 6 h after a block when a motor response was still obtained with a current <0.2 mA versus nerve stimulation with an intensity between 0.3 and 0.5 mA in a pig model.
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Methods
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This project was approved by the Austrian Federal Animal Investigational Committee (GZ 66.011/61-BrGT/2003), and the pigs (10 healthy 12- to 16-wk-old swine) were managed in accordance with the American Physiological Society, institutional guidelines, and Position of the American Heart Association on Research Animal Use, as adopted on November 11, 1984. The pigs were adequately anesthetized, endotracheally intubated, as previously described (7), and subsequently placed in the prone position with both hind legs slightly abducted. Anatomic landmarks were identified for the posterior approach to a sciatic nerve block, and the nerve block was performed with a short beveled (30°), insulated needle (Stimuplex, Pajunk, Geislingen, Germany; 5 cm in length; 22-gauge) and a standard nerve stimulator (Stimuplex HNS 10, Braun, Melsungen, Germany).
After randomization (flipping a coin), the needle was advanced perpendicularly to the skin, whereas the nerve stimulator delivered a rectangular current of 1 mA, 0.3-ms duration, and 1 Hz. As soon as the typical motor response (plantar flexion of the hoof) was obtained, the current was gradually decreased.
In each pig (n = 10), the local anesthetic was injected on one side when a motor response was elicited with a current <0.5 mA but not less than <0.3 mA. On the other side of the same pig (n = 10), the needle was repositioned until the plantar flexion of the hoof was still obtained with a current <0.2 mA. In both groups, 20 mL of plain bupivacaine 0.25% (Bucain, Curasan, Vienna, Austria) was injected during continuing stimulation. Whereas the needle was retracted, 2 mL of Methylen blue dye was injected in the gluteus muscle, and the entrance side of the needle was marked with a suture to facilitate postmortem preparation. Six hours after injection, the pigs were killed, and the sciatic nerves were carefully freed form surrounding tissue on both sides and removed for microscopic examination. This time frame was chosen to allow onset of histological alterations in an acute animal study. Each formalin-fixed and paraffin embedded specimen (4 cm in length) was cut in a series of >200 slices at representative parts. As such, a total of >4000 sections (7 µm) were analyzed. The investigator examining the slides was blinded to treatment groups. When either a well-defined focus or a light diffuse scattering of mononuclear cells was identified in two or more slides, this was judged indicative for an inflammatory response.
The incidence of inflammation signs after 0.3–0.5 mA versus <0.2-mA current stimulation was compared using Fisher’s exact test. We considered a two-tailed value of P < 0.05 statistically significant. Statistical power was calculated to be 82% for the one-sided test and a type I error of 0.05 using nQueryTM software (nQuery, Atlanta, GA).
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Results
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The mean ± sd current required to obtain a motor response of the hoof was 0.4 ± 0.05 mA in the standard current group and 0.15 ± 0.04 mA in the very low current group. Histologic assessment of all sciatic nerves blocked during standard current stimulation revealed no signs of inflammation. In 5 of 10 sciatic nerves blocked during very low current stimulation, lymphocytes and polymorphic granulocytes were found sub-, peri-, and intraneurally (P < 0.003 for comparison of incidence among groups; Fig. 1). In one sciatic nerve specimen of the very low current stimulation group, histologic examination revealed a disruption of the perineurium and multiple nerve fibers (Fig. 2).
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Figure 1. Representative transversal microscopic view of a sciatic nerve 6 h after injection of 20 mL of plain bupivacaine 0.25% when a motor response was elicited with a current <0.2 mA. Arrow indicates lymphocytes and granulocytes with polymorphic nuclei.
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Figure 2. Longitudinal microscopic view of a sciatic nerve blocked with 20 mL of plain bupivacaine 0.25% after obtaining a motor response with a current <0.2 mA. Arrow indicates lymphocytes and granulocytes with polymorphic nuclei; *indicates the region of perineural disruption and damage of nerve fibers.
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Discussion
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In a French study enrolling 42,000 peripheral nerve blocks, neuropathy was observed in 9 patients when the nerve stimulator was used: in 3 cases, an intensity <0.5 mA was documented during the procedure (1). Despite these observational findings, there are no studies demonstrating a relationship between the amount of current used to elicit a motor response before injection of the local anesthetic and histologic alterations of the nerve. In our pilot study in pigs, we detected lymphocytes and polymorphic granulocytes within and around the sciatic nerve after very low current (<0.2 mA) stimulation in 50% of the pigs, whereas this phenomenon was not observed in the group in which a higher current limit was used.
According to Coulomb’s law (stimulation threshold = current/[distance between needle tip and nerve2] – k), the less current that is required to elicit a motor response, the closer the needle’s tip is located to the nerve bundle. Accordingly, muscle twitches obtained with decreasing levels of current correlate with a successful block. In one study of midhumoral block, Carles et al. (8) identified an increased risk of failure for currents >0.6 mA but no further improvement of onset and quality with currents <0.4 mA. Accordingly, the recommendation of Jochum et al. (9) to attempt a motor response with a stimulating current as low as 0.1–0.2 mA might be questioned. The possible relationship between a very low current motor response and inflammation observed in our study may suggest that the needle tip might already have reached the nerve.
Although our study design is very close to common clinical practice, it must be noted that the position of the needle tip was not known. Moreover, all pigs underwent general anesthesia. Accordingly, we were unable to detect clinical signs of a direct needle-to-nerve contact, such as paresthesias, or pain during injection of the local anesthetic. Furthermore, the clinical relevance of the observed signs of inflammation remains unclear. Nevertheless, when comparing our histologic findings with those obtained in post-tooth-extraction oral traumatic neuromas, there is a similar mononuclear inflammatory cell pattern. The authors of the latter studies reported that patients who had histologic signs of inflammation in their lingual nerve specimen also frequently complained about pain and tingling (10,11). However, even if the local inflammation seems minor and may not result in any neurologic impairment, it might have been even more impressive after a prolonged observation period. The observed partial disruption of nerve fibers in one animal is a serious, but not statistically significant event.
In conclusion, microscopic evaluation 6 hours after sciatic nerve stimulation with a current <0.2 mA and subsequent injection of bupivacaine revealed lymphocytes and granulocytes sub-, peri-, and intraneurally in 50% of the pigs. No signs of inflammation were observed when the muscle contraction was induced with a current between 0.3 and 0.5 mA. The association between inflammation and nerve stimulation technique is worthy of further investigation.
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Footnotes
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Supported, in part, by the Austrian National Bank Grant No. 0513, Vienna, Austria, and the Science Foundation of the Tyrolean State Hospitals (TILAK), Tyrol, Austria.
Accepted for publication July 5, 2005.
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References
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Abstract
Introduction
